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Epidermis

Keratinocytes

Keratinocytes are subdivided according to degree of differentiation into the following four types: (1) basal cells, the germinative, epidermal stem cells, which are fixed to the basement membrane by attachment plaques (modified desmosomes) and anchoring filaments (stratum basale); (2) prickle or spinous cells (stratum spinosum), characterized by the presence of keratin and desmosomes and named for the desmosomal intercellular attachments, which appear as spines or prickles connecting cells; (3) granular cells (stratum granulosum), containing numerous large basophilic keratohyalin granules in the cytoplasm; and (4) cornified cells (stratum corneum)—flat, anucleate cells that form the most superficial layer of the epidermis. The stratum corneum is continuously shed at the surface and varies greatly in thickness—it is greatest in the soles and palms.

Melanocytes

Melanocytes are epidermal cells derived from the embryologic neural crest. They produce melanin and are situated singly in the basal layer, appearing as larger clear cells. The dopa reaction, which is positive in the presence of enzymes of the tyrosine–melanin pathway is a histochemical means of identifying melanocytes. Melanocytes have dendritic processes that ramify in the epidermis and transfer melanin to keratinocytes.

Melanocytes can be identified through electron microscopy by the presence of melanosomes, which are membrane-bound ellipsoidal structures containing concentric internal lamellae. Positive staining for S100 protein and melanosomal antigen (HMB45) are useful immunohistochemical markers for melanocytes.

The number of melanocytes in the skin is relatively constant. Skin pigmentation is dependent on the rate of synthesis of melanin, which is governed by (1) racial factors, being greater in dark-skinned races; (2) ultraviolet radiation, which increases melanin synthesis; and (3) hormones—melanocyte-stimulating hormone and adrenocorticotropin both increase melanin pigmentation.

Langerhans Cells

Langerhans cells are clear dendritic cells situated among the cells of the stratum spinosum. They are believed to be antigen-processing cells. They are S100 protein-positive on immunohistochemical studies. On electron microscopy, they lack melanosomes but contain a characteristic organelle known as a Birbeck granule.

Merkel Cells

Merkel cells are neuroendocrine cells that are present in the basal layer of the epidermis. They cannot be recognized in routine histologic sections but can be identified in electron micrographs by the presence of cytoplasmic neurosecretory granules.